Microporous polyolefin membranes are useful as separators for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc. When the microporous polyolefin membrane is used as a battery separator, particularly lithium ion battery separator, the membrane's performance significantly affects the battery's properties, productivity, and safety. Accordingly, the microporous polyolefin membrane should have appropriate permeability, mechanical properties, heat resistance, dimensional stability, shutdown properties, meltdown properties, etc. As is known, it is desirable for the batteries to have a relatively low shutdown temperature and a relatively high meltdown temperature for improved battery safety properties, particularly for batteries exposed to high temperatures during under operating conditions. High separator permeability is desirable for high capacity of batteries. A separator with high mechanical strength is desirable for improved battery assembly and fabrication properties.
The optimization of material compositions, stretching conditions, heat treatment conditions, etc., has been proposed to improve the properties of microporous polyolefin membranes used as battery separators. For example, JP6-240036A discloses a microporous polyolefin membrane having improved pore diameter and a sharp pore diameter distribution. The membrane is made from a polyethylene resin containing 1% or more by mass of ultra-high-molecular-weight polyethylene having a weight-average molecular weight (“Mw”) of 7×105 or more, the polyethylene resin having a molecular weight distribution (weight-average molecular weight/number-average molecular weight) of 10-300, and the microporous polyolefin membrane having a porosity of 35-95%, an average penetrating pore diameter of 0.05-0.2 μm, a rupture strength (15-mm width) of 0.2 kg or more, and a pore diameter distribution (maximum pore diameter/average penetrating pore diameter) of 1.5 or less. This microporous membrane is produced by extruding a melt-blend of the above polyethylene resin and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling at a temperature from the crystal dispersion temperature (“Tcd”) of the above polyethylene resin to the melting point (“Tm”) +10° C., removing the membrane-forming solvent from the gel-like sheet, re-stretching the resultant membrane to 1.5-3 fold as an area magnification at a temperature of Tm of the above polyethylene resin −10° C. or less, and heat-setting it at a temperature from Tcd of the above polyethylene resin to Tm.
WO 1999/48959 discloses a microporous polyolefin membrane having suitable strength and permeability, as well as a uniformly porous surface without local permeability variations. The membrane is made of a polyolefin resin, for instance, high-density polyethylene, having Mw of 50,000 or more and less than 5,000,000, and a molecular weight distribution of 1 or more to less than 30, which has a network structure with fine gaps formed by uniformly dispersed micro-fibrils, having an average micro-fibril size of 20-100 nm and an average micro-fibril distance of 40-400 nm. This microporous membrane is produced by extruding a melt-blend of the above polyolefin resin and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling at a temperature of Tm of the above polyolefin resin −50° C. or higher and lower than Tm, removing the membrane-forming solvent from the gel-like sheet, re-stretching it to 1.1-5 fold at a temperature of Tm of the above polyolefin resin −50° C. or higher and lower than Tm, and heat-setting it at a temperature from Tcd of the above polyolefin resin to Tm.
WO 2000/20492 discloses a microporous polyolefin membrane of improved permeability which is characterized by fine polyethylene fibrils having Mw of 5×105 or more or a composition comprising such polyethylene. The microporous polyolefin membrane has an average pore diameter of 0.05-5 μm, and the percentage of lamellas at angles θ of 80-100° relative to a membrane surface is 40% or more in longitudinal and transverse cross sections. This polyethylene composition comprises 1-69% by weight of ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 7×105 or more, 98-1% by weight of high-density polyethylene, and 1-30% by weight of low-density polyethylene. This microporous membrane is produced by extruding a melt-blend of the above polyethylene or its composition and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling, heat-setting it at a temperature from Tcd of the above polyethylene or its composition to (Tm +30° C.), and removing the membrane-forming solvent.
WO 2002/072248 discloses a microporous membrane having improved permeability, particle-blocking properties and strength. The membrane is made using a polyethylene resin having Mw of less than 380,000. The membrane has a porosity of 50-95%, an average pore diameter of 0.01-1 μm. The microporous membrane has a three-dimensional network skeleton formed by micro-fibrils having a average diameter of 0.2-1 μm connected to each other throughout the overall microporous membrane, and openings defined by the skeleton to have an average diameter of 0.1 μm or more and less than 3 μm. This microporous membrane is produced by extruding a melt-blend of the above polyethylene resin and a membrane-forming solvent through a die, removing the membrane-forming solvent from a gel-like sheet obtained by cooling, stretching it to 2-4 fold at a temperature of 20-140° C., and heat-treating the stretched membrane at a temperature of 80-140° C.
WO 2005/113657 discloses a microporous polyolefin membrane having suitable shutdown properties, meltdown properties, dimensional stability, and high-temperature strength. The membrane is made using a polyolefin composition comprising (a) a polyethylene resin containing 8-60% by mass of a component having a molecular weight of 10,000 or less, and an Mw/Mn ratio of 11-100, wherein Mn is the number-average molecular weight of the polyethylene resin, and a viscosity-average molecular weight (“Mv”) of 100,000-1,000,000, and (b) polypropylene. The membrane has a porosity of 20-95%, and a heat shrinkage ratio of 10% or less at 100° C. This microporous polyolefin membrane is produced by extruding a melt-blend of the above polyolefin and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling, removing the membrane-forming solvent, and annealing the sheet.
With respect to the properties of separators, not only permeability, mechanical strength, dimensional stability, shutdown properties and meltdown properties, but also properties related to battery productivity such as electrolytic solution absorption, and battery cyclability, such as electrolytic solution retention properties, have recently been given importance. In particular, electrodes for lithium ion batteries expand and shrink according to the intrusion and departure of lithium, and an increase in battery capacity leads to larger expansion ratios. Because separators are compressed when the electrodes expand, it is desired that the separators when compressed suffer as little a decrease as possible in electrolytic solution retention.
Moreover, even though improved microporous membranes are disclosed in JP6-240036A, WO 1999/48959, WO 2000/20492, WO 2002/072248, and WO 2005/113657, further improvements are still needed particularly in membrane permeability, mechanical strength, heat shrinkage resistance, compression resistance, and electrolytic solution absorption properties. It is thus desired to form battery separators from microporous membranes having improved permeability, mechanical strength, heat shrinkage resistance, compression resistance and electrolytic solution absorption.